Ok, i may sound stupid, but when i download a mod for a game, (ex. Unreal Tournament 2003), will it completley delete Unreal Tournament 2003? or will it just make a whole different thing and while my Unreal still exists?

Continuing on from what Falling Blue has said,a mod is basically an update of a game updated for visual or bug repairing reasons,to return the game to its original settings you need to completly uninstall the game and delete the mod file - which should be located in a folder/file external to the main games folder.When you reinstall the game it should return to its original settings. The orginal game will be unaffected regardless of the number of mods you install onto it.